The present exemplary embodiments relate to printing or copying systems and, in particular, printing devices which utilize an intermediate transfer service such as a transfer drum which is intended to engage a receiving medium such as paper for imparting a desired image from the drum to the medium. The subject embodiments are especially applicable to printing devices which utilize a supply of colored inks to be communicated to a print head for document printing wherein the inks are supplied as solid ink sticks which must be heated to a liquid form before communication to the print head. Such systems are commercially available under the PHASER® mark from Xerox Corporation. The heating of the ink to effect the solid-to-liquid phase change is usually associated with corresponding heating of other components of the assembly, such as the drum and the medium itself before engagement with the drum. The subject embodiments are particularly directed to the structure and method of operation of the heater for the preheating of the medium prior to the transfer process.
All printers and copying machines have to be designed with an appreciation that at some time a power control failure may occur within the system and that such failure should not expose an operator or a repairman to a dangerous situation such as exposure to electrical shock or thermal burning. Indeed, a typical safety requirement (UL required) is that any component that has line voltage and is accessible by a user or operator must have enough insulation protection to have a dielectric strength of at least 3 KV between the user accessible parts, ground plane, secondary circuits and the line. This safety regulation must be met not only by new componentry but also by componentry that has been exposed to thermal run-away conditions that can damage the insulation.
The subject embodiments concern the operation and assembly of a medium preheater in a printing system, which preheater is a typical component subject to the above safety requirements. The conventional construction of such a heater involves a pattern of heat traces laminated to a metallic support plate. The support plate is disposed to engage the medium for the heating of the medium immediately prior to its engagement with the intermediate transfer drum and the imparting of a desired image from the drum to the medium. Laminating of the heat traces to the support plate involves insulating a layer therebetween which under normal use conditions would satisfy the 3 KV dielectric strength requirement. However, the heat traces in such a system are typically capable of reaching relatively extreme temperatures (about 1200° C.) which is a temperature that is easily capable of burning away an insulating layer between the heat traces and the support plate. The plate would then function as a ground plane for the electrical supply to the traces thereby grounding the line voltage to the component. Such an occurrence would fail to meet the above-referenced safety requirements.
There is a need for a heater assembly which is properly fused to interrupt this supply of electrical power to the heater in the event of a thermal run-away and at a point in time prior to the thermal run-away causing unacceptable damage to the pattern of heat traces themselves and/or the insulating layer separating the heat traces from the metallic support plate.
The present exemplary embodiments satisfy this need as well as others to provide a power control system for medium heaters in phasing printing systems that can provide the desired safety protection against power control failures that may cause thermal run-aways in the system.